Chlamydia trachomatis (Ct) is the most common cause of bacterial sexually transmitted disease worldwide. In previous work, we identified epitopes in the major outer membrane (MOMP) that are recognized by CD4+ T cells in the context of HLA class II molecules and others that are recognized by CD8+ T cells in the context of HLA class I molecules. Most recently, we used HLA A2 tetramers to identify MOMP-specific CD8+ T cells in blood of Ct-infected individuals and found that they killed Ct infected epithelial cells. We will extend these studies to other individuals using HLA class I tetramers made with other allotypes. To identify and characterize MOMP-specific CD4+ T cells isolated from blood, we will create HLA class II tetramers using MOMP epitopes that are recognized by CD4+ T cells. We will characterize both CD8 and CD4 MOMP-specific T cells with regard to cell surface proteins that are involved in homing (e.g. integrins and chemokine receptors), function (e.g. cytokines, perforin) and state of differentiation (e.g. HLA-DR, CD62L) by using multi-color flow cytometry with monoclonal antibodies against cell surface and intracellular proteins. Comparisons of lymphocytes from the blood vs. sites of infection will provide information on how representative blood-derived samples are of cells at infected sites. Lastly, the ability of MOMP-specific CTL to limit the increase of infectious Ct in infected genital tract cells and the effects of mutations in CTL epitopes on such limitation will be determined. The ability to detect and characterize human anti-Chlamydial T cells with tetramers and cellular reagents of defined HLA- and epitope specificity would be useful for vaccine development, especially since most of the epitopes are in conserved regions of MOMP.